Infants and young children are susceptible to gastroesophageal reflux. When a baby feeds, milk passes from the mouth to the stomach through the esophagus. The esophageal sphincter acts as a one-way valve to prevent milk from flowing back in the opposite direction. Reflux occurs when food in the stomach flows back up into the mouth, causing the baby to vomit or spit up. Usually, reflux in infants and young children is rarely serious and can go away on its own. However, if the baby is over 18 months old and still has this condition, it can be considered abnormal. In this case, the doctor may order an ultrasound to diagnose gastroesophageal reflux.